I. Field of the Invention
The present inventioln relates to the use of molten metal baths to treat waste streams, coal, garbage and the like.
II. Descripton of the Prior Art
There have been many attempts made at harnessing the power of molten metal baths to process coal, hydrocarbons, garbage, tires and the like. Molten iron is almost a universal solvent, able to rapidly decompose and/or convert a wide variety of difficutlt materials. As an example, consider coal processing. Coal is an inexpensive and readily available fuel. Unfortunately it is a dirty fuel. The coal can be burned, but doing so creates many environmental problems, including dust, NOx, SOx, and CO emissions.
Because coal is such a difficult feed, a summary of some coal conversion art follows. It should, however, be noted that the present invention is not limited to coal conversion. Coal, perhaps the most difficult fuel to process rather than butn, provides a good limiting case study oln hydrogen deficientt feeds, so a review of coal conversion art is instructive. This is followed by a review of art at producing hydrogen from clean fuels such as methane. As will be seen, molten metal processing of these hydrogen-deficient, e.g., coal, and hydrogen-rich, e.g., methiane, feeds catise problems.
Coal can be converted at great expense to for valuable clean fuels or at lesser expense to cheap, dirty fuels. The high expense route to clean fuels involves gasification followed by Fisceher Tropsch synthesis. This approach works, and has been used commercially by those countries without a secure source of oil. The use of coal to form water gas or other low grade fuel which can be burned (with much of the contaminants still inl the gas) is a low tech, environmentally unsuitable way to get a gaseous fuel from a difficult solid.
One promising new approach is use of molten metal baths to process coal. Coal dissolution is rapid. Ash, if present, will from slag. The slag must be removed but, at least, the ash does not clog filters or slow coal processing. Hydrogen in the coal is rapidly released as hydrogen and usually just as rapidly burned because oxygen addition usually proceeds in lock step with carbon burning, and in the same bath of molten metal receiving coal.
U.S. Pat. Nos. 4,187,672 and 4,244,180 (Rasor) provide all example of this approach. Coal was fed to a bath of molten iron, and air introduced into the reactor to react with dissolved carbon to for hot fuel gas.
What is common to all molten metal processes converting coal, or other hydrogen-deficient fuel, is the processes generally produce excess heat and/or a dirty fuel gas product. In contrast, molten metal processing of clean fuels produces hydrogen or, at least, a cleaner fuel gas, and some heat input (or hydrogen combustion) is needed for heat balance.
A two-zone approach was used for methane conversion in U.S. Pat. No. 1,803,221. By separating the feed addition zone from the oxygen addition zone, it was possible to obtain a relatively pure hydrogen product from one zone.
Altough not primarily a coal conversion case, a promising approach involved use of a two-zone molten metal apparatus as disclosed in Miller et al., U.S. Pat. No. 5,435,814, which is incorporated by reference. A hydrogen- and carbon-containing feed was added to one pair of a circulatinig bath of molten iron, while oxygen or air was added to another part of the bath. A baffle separated the molten iron bath into two zones, a reducing zone (into which feed was added) and an oxidizing zone (into which O.sub.2 was added). This allowed a heat-balanced operation to be achieved. Significant amounts of hydrogen-rich gas were produced in the reducing side via endothermic reactions while exothermic burning of dissolved carbon in the oxidizing side supplied the heat needed for the process. This isolation of feed dissolution (or more accurately, feed dissociation) from burning of dissolved carbon in a continuously cirlculating bed greatly improved the flexibility of the process and the value of the products.
Because the process in '814 was heat balanced, some care had to be taken to ensure that the process would operate continuously without external heating or cooling. The continuous process had to be thermally balanced to make sure that the bed did not get too hot or too cold.
Clean light feeds, such as methane, when charged to a continuous process generated large amounts of hydrogen (highly exothiermic) relative to the amount of carbon dissolved in the bath. Hydrogen-deficient feeds such as coal potentially added very little hydrogen to the bath, but large amounts of carbon. In short, if one wanted to get useflil products from both zones, it was difficult to operate with very clean, hydrogen-rich feeds (too endotherimic) or with dirty or hydrogen-deficient feeds (too exothermic).
Patentees have devised several ways to overcome this problem, mixing light and heavy feed, or adding CO or steam to the oxidizing side to promote other reactions which would keep things in heat balance.
While these approaches helped to a considerable extent, they all were somewhat restrictive in practice. If a refiner, or waste processor, had to dispose of some asphaltic feed, the "mixing" solution would make the feed lighter (or richer in hydrogen) by adding some lighter material such as methane. The methlane/asplhaltic "blend" would then be processed in the molten metal reactor, yielding a hydrogen product stream of intermediate purity. The hydrogen would be purer than could be obtained when processing an asphalt feedstock, but not as pure as when processing a methane feedstock.
We wanted more freedom than this in operating the unit. We wanted freedom to process both hydrogen-deficient and hydrogen-rich feeds while maintaining a heat-balanced operation. We discovered that it was possible to keep a heat-balanced operation while processing different quality feeds without mixing the feeds provided the feeds were charged to separate zones in the molten metal bath (or by short tenn blocked feed operation of two baths or even a single bath), and provided that the hydrogen-rich feed was charged to a zone uncontaminated with dirty feed and free of added oxygen.
By providing three separate zones, or rapidly cycling between three modes of operation in a single bath, we were able to obtain good hydrogen purities (during the relatively short periods when hydrogen-rich feed was processed) and intermediate hydrogen purities (when poor quality feed was processed). All this could be done in a heat-balanced unit.
When practiced in a continuous unit, such as that described and claimed in our parent patent, the hydrogen content in the gas produced in each dissociation zone is primarily indicative of the feed properties to each zone, while the carbon level increases slowly (for methane) or quickly (for SDA bottoms). The net amount of carbon added by both zones was appropriate for the amount of coke burned out of the molten metal bath in the oxidation zone.
When practiced in a multi-bed, e.g., three-bed unit, the feed and product lines associated with each zone will periodically shift from feed 1, to feed 2 to an oxidant.
When methane is feed 1, a relatively high purity hydrogen stream will be withdrawn from zone 1, but hydrogen purity will rapidly drop when a lower hydrogen content feed is charged to this zone.
When practiced in a single-vessel reactor, where the same vessel serves sequentially to process feed 1, then process feed 2, and then as a dissolved carbon-burning vessel, the same steps occur. The single vessel approach involves the lowest capital cost for a small unit, but may require means for dealing with abrupt changes in hydrogen purity and some periods of no hydrogen production. Compressors and gas storage facilities can be used to deal with the sudden swings in production.
By using two zones for feed dissociation, and a third zone for combustion of dissolved carbon, we were able to obtain a heat-balanced operation with disparate stocks, with peak hydrogen purity rathler than average hydrogen purity being achieved, at least, some of the time.